Crucible having a doped upper wall portion and method for making the same

ABSTRACT

A fused glass crucible includes a collar of doped aluminum silica that defines uppermost and outermost surfaces of the crucible. The melt line that defines the surface of molten silicon in the crucible may be substantially at the lower end of the collar or slightly above it. Crystallization of the collar makes it hard and therefore supports the remaining uncrystallized portion of the crucible above the melt line. The melt line may also be below the lower end of the collar, especially if the melt is drawn down or poured early in the process. Because there is little or no overlap or because the overlap does not last long, the doped aluminum collar is not damaged by the heat of from the melt.

SUMMARY OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of silica crucibles and moreparticularly to a silica crucible having a doped layer formed in thewall.

2. Background of the Invention

The Czochralski (CZ) process is well-known in the art for production ofingots of single crystalline silicon, from which silicon wafers are madefor use in the semiconductor industry.

In the CZ process, metallic silicon is charged in a silica glasscrucible housed within a susceptor. The charge is then heated by aheater surrounding the susceptor to melt the charged silicon. A singlesilicon crystal is pulled from the silicon melt at or near the meltingtemperature of silicon.

In addition to the CZ process, fused silica crucibles are used to meltmetallic silicon, which is then poured—from a nozzle formed into thecrucible—into a mold to create a polycrystalline silicon ingot, which isused to make solar cells. As with the CZ crucible, a heater surrounds asusceptor, which holds the crucible.

When fused glass crucibles are so used, metallic silicon in the cruciblemelts—at least in part—as a result of radiant heat transmitted by theheater through the susceptor and crucible. The radiant heat melts thesilicon in the crucible, which has a melting point of about 1410 degreesC., but not the crucible. Once the silicon in the crucible is melted,however, the inner surface of the crucible beneath the surface of themolten silicon is heated to the same temperature as the molten siliconby thermal conduction. This is hot enough to deform the crucible wall,which is pressed by the weight of the melt into the susceptor.

The melt line is the intersection of the surface of the molten siliconand the crucible wall. Because the wall above the melt line is notpressed into the susceptor by the weight of the melt, i.e., it isstanding free, it may deform. It is difficult to control the heat tomelt the silicon, and keep it molten, while preventing the wall abovethe melt line from sagging, buckling or otherwise deforming. Maintainingprecise control over the heat slows down the CZ process and thusthroughput of silicon ingots.

It is known in the art to form a fused crucible with doped silica in theouter layer. The element used to dope the silica is one that promotescrystallization, such as aluminum, when the crucible is heated.Crystallized silica is much stronger than fused glass and will notdeform as a result of heat in furnaces of the type used in the CZ andsimilar processes.

One such known approach dopes the outer layer of a crucible withaluminum in the range of 50-120 ppm. Relatively early in the course of along CZ process, the outer wall crystallizes as a result of the aluminumdoping. The crystallized portion is more rigid than the remainder of thecrucible and therefore supports the upper wall above the melt line.

This prior art approach produces at least two kinds of problems,depending on the level of doping. First, the doping level must be highenough to create a rigid outer wall that supports the upper wall abovethe melt line. If the doping level is too low, the wall is subject todeformation in a manner similar to an undoped crucible. But when thedoping level is high enough to support the upper wall, that portion ofthe wall beneath the melt line is subject to very high heat during theCZ process. This forms a very thick crystalline layer below the meltline. As a result of the prolonged heat and thick crystalline layer, thewall beneath the melt line may crack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are cross-sectional, highly schematic, side views of a moldshowing sequential stages for forming a crucible of the type having afunnel at the lower end thereof.

FIG. 4 is a cross-sectional view of a crucible so formed.

FIG. 5 is a cross-sectional view of an alternative crucible formedaccording to the present invention in use during a CZ process.

FIG. 6 is a cross-sectional view of the crucible of FIG. 4 in use duringa process for making solar cells.

DETAILED DESCRIPTION

Indicated generally at 10 in FIG. 1 is a system for making a fused glasscrucible. The system includes a crucible mold 12 that is rotatable on avertical axel 14. Mold 12 includes a generally horizontal surface 14 onwhich a bottom portion of a crucible is formed, as well be seen. Themold also includes a generally upright surface 16 against which a wallportion of the crucible is formed. In FIG. 1, system 10 is configured toform a crucible of the type having a nozzle at a lower end thereof. Tothis end, a graphite plug 18 is positioned in a lower end of the mold toform a passageway that communicates with a nozzle (not shown) that isattached to the crucible after it is fused. For the details ofmanufacturing a crucible having such a nozzle, reference is made to U.S.patent application Ser. No. 11/271,491 for a Silica Vessel with Nozzleand Method of Making, filed Nov. 9, 2005, which is hereby incorporatedherein by reference for all purposes.

System 10 includes a bulk grain hopper 20 and a doped grain hopper 22.The flow of grain from each hopper is controlled by regulating valves24, 26, respectively. A feed tube 28 introduces flow of silica graininto mold 12 from either one of or both of the hoppers depending uponhow valves 24, 26 are set. Feed tube 28 is vertically movable into andout of mold 12. This facilitates selectively depositing grain on uprightsurface 16 and on generally horizontal surface 14, as well be furtherexplained. A spatula 30 is also vertically movable and in addition ishorizontally movable to shape grain in mold 12 as it rotates.

Consideration will now be given to how system 12 is used to make acrucible. First, hopper 20 is loaded with bulk silica grain 32. Andhopper 22 is loaded with aluminum-doped silica grain 34. Silica grain 34may be doped with aluminum in the range of about 85-500 ppm.

Next, mold 12 is rotated at a rate of about 100 rpm, feed tube 28 ispositioned as shown in FIG. 1, and valve 26 is opened to begindepositing doped grain 34 in a band or collar 36 about the perimeter ofmold 12. The feed tube is moved vertically to deposit doped grain asshown. The rotation rate is high to keep the doped grain in collar 36above a predetermined level on generally upright surface 16. If therotation rate is too low, doped grain falls into lower portions of themold, which is undesirable. In the present embodiment, the radiallyouter surface of collar 36 comprises the outermost portion of theuppermost part of the crucible wall. The doped grain that forms thecollar is deposited in a layer that has a thickness (measured along aradial axis of mold 12) that is defined by the position of spatula 30.This thickness may have a range of about 0.7-2.0 mm in the fully formedcrucible. As will be seen, there is an outermost layer of silica grainthat is not fused. This prevents burning of the mold and makes it easierto remove the crucible from the mold. The thickness of this unfusedgrain must be taken into account to provide the 0.7-2.0 mm thickness inthe finished product.

After collar 36 is laid down as described above, valve 26 is closed, andvalve 24 as opened, as shown in FIG. 2. In addition, the rate ofrotation of mold 12 is reduced to 75 rpm. This permits some of the bulkgrain 32 to fall to the lower portion of mold 12. As bulk silica grainfeeds from hopper 20 out of feed tube 28, the feed tube moves verticallyto coat the side and bottom of the mold with a layer 38 of bulk grainsilica as shown. Spatula 30 shapes the bulk grain layer into the form ofa crucible. As can be seen, layer 38 covers substantially all of collar36. Graphite plug 18 defines an opening through layer 38 in the shape ofthe plug.

With reference to FIG. 3, after the silica grain crucible is defined inmold 12 as shown in FIG. 2, spatula 30 and feed tube 28 are withdrawn.Electrodes 40, 42 are vertically movable into and out of the interior ofmold 12. The electrodes are attached to a DC power supply 46 that canapply power to the electrodes in a selectable range between about 300KVA and 1200 KVA. When sufficient power is supplied to the electrodes,an extremely hot plasma ball forms around the electrodes. The heat sogenerated creates a fusion front that fuses the silica grain beginningat the inner surface of the formed crucible and proceeding to the outersurface. This fusion front fuses most of layer 38 and the collar 36 ofdoped silica grain but stops—as a result of stopping the application ofpower to electrodes 40, 42—before it fuses an outermost unfused layer 49of grain that includes both bulk silica grain 38 and doped silica grain36. As previously mentioned, the depth of the grain deposited into mold12 must take into account this unfused layer 49 so that a depth of thefused doped grain 36, as shown in FIG. 4, is in the range of 0.7-2.0 mm.A unitary fused glass crucible 50 is shown in FIG. 4 after it is removedfrom mold 12 and graphite plug 18 has been removed.

It can be seen that an upper portion of crucible 50 has been cut off toproduce a flat upper rim 52. This provides a crucible of a predeterminedheight and also provides a flat upper rim. As can be seen, in FIG. 4,collar 36 provides an outermost and uppermost portion of crucible 50.After the upper portion of the cut is made, collar 36—in the presentembodiment—extends about 50 mm downwardly from rim 52. It should beappreciated, however, that collar 36 could be formed to extend muchfurther down the crucible—as much as ⅔ or ⅓ of the way down thusproviding a much taller collar. As will be described shortly, a shortercaller is preferred.

Turning now to FIG. 5, indicated generally at 54 is a crucible in use ina CZ process. Crucible 54 is made in substantially the same manner ascrucible 50 except that it does not have an opening in a lower portionthereof. This is accomplished simply by using a mold having a continuoussmooth lower surface and omitting use of a graphite plug, like plug 18.Crucible 54 includes an aluminum doped collar 56, which is formed asdescribed above in connection with crucible 50. Like crucible 50,crucible 56 has been cut along a plane at right angles to itslongitudinal axis. This produces a substantially flat rim 58.

Crucible 54 is supported in a susceptor 60 that is inside a furnace (notshown). The susceptor is surrounded by a heater 62. Crucible 54 has beencharged with metallic silicon that has melted, which is now referred toas the melt 64, in response to heat produced by heater 62 inside thefurnace. A single silicon seed crystal 61 is held by a holder 63, whichslowly draws seed crystal 61 from the molten silicon in accordance withthe CZ process. A crystalline ingot 65 forms, also in accordance withthe CZ process, on the lower end of seed crystal 61. Melt line 66 isdefined about the perimeter of crucible 54. The melt line progressivelylowers as ingot 65 forms and is pulled from melt 64.

The melt 64 is at a temperature of about 1400 degrees C. As a result,the surface of crucible 54 beneath the melt line is also at thattemperature. Even though the heat from the melt makes the crucible belowmelt line 66 very soft, the weight of the melt presses the crucible intosusceptor 60 thus preventing any deformation of crucible 54 below meltline 66. As the metallic silicon melts, the heat begins to crystallizecrucible 54 in collar 56 as a result of the aluminum doped siliconwithin the collar. The portion of the crucible that is crystallized ishardened. This creates a relatively rigid crystalline ring or collararound the crucible, which stabilizes the portion of the crucible wallthat is not crystallized. In other words, the rigid collar prevents thesofter uncrystallized wall above the melt line from collapsing orotherwise deforming even as melt line 66 lowers to the bottom of thecrucible.

Finally, crucible 50 is shown in use in FIG. 5. It also is held in asusceptor 68. Likewise a heater 70 surrounds the susceptor 68 with allof the structure shown in FIG. 6 being contained within a furnace (notshown). Silicon melt 72 was formed by melting metallic silicon incrucible 50 by heating it with heater 70 in the furnace. A nozzle 74,which was formed with graphite plug 18, on the lower portion of crucible50 is plugged during while the silicon is melted. Once fully molten, theplug is removed, and melt 72 pours through nozzle 74—as shown in thedrawing—into molds (not shown) that are used to make solar cells.

As with the crucible of FIG. 5, the FIG. 6 crucible walls are supportedas a result of the crystalline ring formed when collar 36 begins tocrystallize early in the CZ process. As a result, the walls of thecrucible are supported above the melt line.

It should be appreciated that the aluminum-doped collars, like collars36, 58, can be formed so that the lower portion thereof is substantiallyat or slightly above the melt line when the crucibles are used. Or theymay be slightly below the melt line—at least at the beginning of the CZprocess. A good position for the lower end of the collar is less thanabout 5% of the crucible height below the melt line.

The following examples demonstrate the advantages of the invention.

EXAMPLE A

A crucible like crucible 50 was formed that has a height of 400 mm, 270mm inner diameter, and 10 mm wall thickness. In this example thecrucible was doped with 100 ppm aluminum to form a collar, like collar36 that extends 150 mm down from rim 52. The collar is 1.4 mm thick anddefines an outermost and uppermost surface of the crucible as shown inthe drawing. A charge of 120 kg metallic silicon was charged and kept inthe crucible for 120 hours without problems.

EXAMPLE B

A crucible like crucible 50 was formed that has a height of 400 mm, 270mm inner diameter, and 10 mm wall thickness. In Example B the cruciblewas doped with 500 ppm aluminum to form a collar, like collar 36 thatextends 50 mm down from rim 52. The collar is 1.6 mm thick and definesan outermost and uppermost surface of the crucible as shown in thedrawing. A charge of 120 kg metallic silicon was charged and kept in thecrucible for 120 hours without problems.

EXAMPLE C

A crucible like crucible 50 was formed that has a height of 400 mm, 270mm inner diameter, and 10 mm wall thickness. In this example thecrucible was doped with 100 ppm aluminum to form a collar, like collar36 that extends 310 mm down from rim 52, which is substantially all ofthe generally upright outer wall of the crucible. The collar defines anoutermost and uppermost surface of the crucible as shown in the drawing.A charge of 120 kg metallic silicon was charged and in the crucible. Inthis example, the melt overlaps substantially with the collar. Putdifferently, the melt line was substantially above the lower edge of thecollar. After 50 hours of holding the melt, the crucible showed crackingbetween the substantially upright wall portion and the substantiallyhorizontal bottom portion. This cracking results from the melt being inclose proximity to the doped, and therefore crystallized, collar.

Although the examples each use aluminum as a dopant, it should beappreciated that the invention could be implemented with any dopant thatpromotes crystallization, e.g., Barium.

As can be seen, when the doped portion and the melt do not overlap, oroverlap only slightly, the problems associated with the prior art fullydoped outer crucible wall can be avoided. In addition, when the processuse is known, i.e., how much silicon will be charged in the crucible andhow quickly the melt will be drawn down, a crucible can be designed inwhich there is overlap between the collar and the melt, but only for afew hours, not enough to damage the crucible, during the early stages ofthe process. As a result, the problems associated with the prior art canbe avoided even where there is overlap of the melt and the doped collarin the early stages of the process.

While the invention has been disclosed in its preferred form, thespecific embodiments thereof as disclosed and illustrated herein are notto be considered in a limiting sense. Indeed, it should be readilyapparent to those skilled in the art in view of the present descriptionthat the invention can be modified in numerous ways. The inventorregards the subject matter of the invention to include all combinationsand subcombinations of the various elements, features, functions and/orproperties disclosed herein.

1. A method for making a fused glass crucible comprising: rotating acrucible mold having a generally horizontal surface for forming a bottomportion of the crucible and a generally upright surface for forming awall portion of the crucible; depositing doped silica grain on an upperportion of the upright surface of the rotating crucible mold; depositingbulk silica grain on a lower portion of the upright surface and on thedoped silica grain; depositing bulk silica grain on the generallyhorizontal surface of the mold to form the bottom portion of thecrucible; and fusing substantially all of the grain.
 2. The method ofclaim 1 wherein the method further comprises preventing bulk silicagrain from being deposited on the upright surface above the doped silicagrain.
 3. The method of claim 1 wherein depositing doped silica grain onthe upper portion of the upright surface comprises depositing aluminumdoped silica grain.
 4. The method of claim 3 wherein depositing aluminumdoped silica grain comprises depositing silica grain doped with about 85to 500 ppm aluminum.
 5. The method of claim 1 wherein the method furtherincludes cutting off an uppermost portion of the fused glass crucibleand wherein depositing doped silica grain on an upper portion of theupright surface of the rotating crucible mold comprises depositing dopedsilica grain on the upright surface to form a doped outer layer on thecut crucible that extends from the crucible rim to a level that is aboveabout ⅔ of the way down the crucible wall portion.
 6. The method ofclaim 5 wherein the doped outer layer on the cut crucible that extendsfrom the crucible rim to a level that is about ⅓ of the way down thecrucible wall portion.
 7. The method of claim 1 wherein the methodfurther includes cutting off an uppermost portion of the fused glasscrucible and wherein depositing doped silica grain on an upper portionof the upright surface of the rotating crucible mold comprisesdepositing doped silica grain on the upright surface to form a dopedouter layer on the cut crucible that extends from the crucible rim about50 mm down the crucible wall portion.
 8. The method of claim 1 whereindepositing doped silica grain on an upper portion of the upright surfaceof the rotating crucible mold comprises depositing doped silica grain toa depth of about 0.7 mm to 2.0 mm.
 9. The method of claim 1 wherein thecrucible is used to hold molten silicon up to the level of a melt linedefined by the juncture of the upper surface of the molten silicon andthe wall portion of the crucible and wherein the method furthercomprises: cutting off an uppermost portion of the fused glass crucible;and wherein depositing doped silica grain on an upper portion of theupright surface of the rotating crucible mold comprises depositing dopedsilica grain on the upright surface to form a doped outer layer on thecut crucible that extends from the crucible rim down the crucible wallportion to substantially the melt line.
 10. The method of claim 9wherein the doped outer layer on the cut crucible extends from thecrucible rim down the crucible wall portion to a level slightly belowthe melt line.
 11. The method of claim 10 wherein the doped outer layeron the cut crucible extends from the crucible rim down the crucible wallportion to a level that is less than about 5% of the crucible heightbelow the melt line.
 12. The method of claim 9 wherein the doped outerlayer on the cut crucible extends from the crucible rim down thecrucible wall portion to a level slightly above the melt line.
 13. Amethod for making a fused glass crucible comprising: depositing a layerof doped silica grain to form a collar that defines an outer portion ofan upper portion of a crucible wall; depositing a layer of bulk silicagrain over the collar to form the remainder of the crucible wall;depositing a layer of bulk silica grain to form the bottom of thecrucible; and fusing substantially all of the silica grain.
 14. Themethod of claim 13 wherein depositing a layer of doped silica grain toform a collar that defines an outer portion of the upper portion of thecrucible wall comprises depositing a layer of doped silica grain to formthe outermost portion of the crucible wall.
 15. The method of claim 14wherein depositing a layer of doped silica grain to form a collar thatdefines an outer portion of an upper portion of the crucible wallcomprises depositing a layer of doped silica grain to form the uppermostportion of the crucible wall.
 16. The method of claim 13 wherein themethod further comprises doping the doped silica grain with a dopantthat promotes crystallization in the early stages of a CZ process whenthe crucible is so used.
 17. A fused glass crucible comprising: agenerally upright substantially cylindrical crucible wall formed fromfused bulk silica grain; a generally horizontal crucible bottom formedfrom fused bulk silica grain, the bottom being joined with the lower endof the crucible wall; a substantially cylindrical collar formed fromfused doped silica grain contained in the wall, the collar being locatedat an upper portion of the wall; and a lower portion of the wallextending between the bottom and the collar that is free from dopedsilica grain.
 18. The fused glass crucible of claim 17 wherein thecollar is contained within the outermost portion of the wall.
 19. Thefused glass crucible of claim 18 wherein the collar is contained withinthe uppermost portion of the wall.
 20. The fused glass crucible of claim19 wherein the collar has a height of about 50 mm.
 21. The fused glasscrucible of claim 17 wherein the collar extends from the rim of thecrucible about ⅔ of the way down the wall.
 22. The fused glass crucibleof claim 17 wherein the collar extends from the rim of the crucibleabout ⅓ of the way down the wall.
 23. The fused glass crucible of claim17 wherein the doped silica grain comprises aluminum doped silica grain.24. The fused glass crucible of claim 23 wherein the doped silica grainis doped with aluminum in the range of about 85 ppm to 500 ppm.
 25. Thefused glass crucible of claim 17 wherein the collar has a radial depthof about 0.7 to about 2.0 mm.
 26. The fused glass crucible of claim 25wherein the collar has a height of about 50 mm.